Revert "Warn on account creation when username is adjusted"

This reverts commit a0c72523c7a2c9ca16329f903ca6b269ccccb787.
which has terrible effects on mobile.

Most browser inputs on a mobile phone default to lowercase meaning
a username with a lowercase starting character will always trigger the
warning.

This means the user will have to re-enter their passwords, fill out the captcha
again and their email again (as all these are sensitive and not retained when
reloading the form with the warning)

If this is truly wanted on desktop, more thought it needed for the mobile use
case.

Bug: 61416
Change-Id: I8b93e5a4ca687f87305ced4343dcb28ca2909939